It depends on what you will do with the information.
If you want to ensure that you don't exhaust the supply, call for a
refill and note how much it took to bring it to the full condition.
Over time, you will be able to estimate your usage by month.
If you want to move it or dispose of it, pick it up and weigh it. The
fuel is about 4# per gallon, then add the weight of the empty tank.
Probably weighs less than 2 tons full (4000#).
The use of water to determine the level of the liquid fuel in the tank
could be useful, though the tank geometry will distort the result: the
first two inches of use corresponds to more fuel than the last two
inches since the vessel is curved.
wrote:

One way is to dump (maybe a lot of) ice water on the tank.
Assuming the dew point is above 32F, you'll see a line of dew where
the gas level is. The liquid propane will supply more heat to the
tank and there won't be dew there, but the gaseous propane on top
won't absorb as much and the tank will cool causing condensation.
I've seen LCD strips for 20lb cooking cylinders that would on the
same principle.
Of course, you could weigh it. ;-)

Accurately - weigh it and compare to the tare weight that should
hopefully be listed on the tank data plate.
Roughly, get one of those liquid crystal stick on propane tank gauge
things and keep moving it down the side of the tank (doing the hot water
thing) until you find the LP level. You'll have to watch extra closely
since the tank wall will be thicker than the wall on a 20# tank. Just
remember that other than the half full point, the scale is not linear
due to the cylindrical shape of the tank.
Pete C.

You don't need to lift it very high. With a crane scale attached to the
crane or forklift you only need to get the tank off the ground, 1/4"
would be enough.
Actually I expect that a standard floor jack and a set of electronic
scales as used for race cars would do nicely. You'd only need to lift
each end of the tank a few inches to slip the scales under the feet.
Pete C.

That would still work. The buyer or the seller can fill up the tank,
then you can reimburse the seller if he paid for the fillup, and take
possession of all of it.
You'r e buying the house next to it, yes? If you are just out there
buying propane as a business, you should probably find another guy who
does this and learn all the details.

You (OP) are the one who asked about buying your tank from the
supplier, rather than have the propane metered out and billed monthly. I
agree that the thing to do is have the supplier fill it up, then pay
them for 400 gallons. Note: 400 gallons is not a mistake-- propane tanks
are filled to 80% of capacity to allow for expansion, thus a properly
filled 500 gallon tank will hold 400. Also. now (summer) is the time to
do this as the price is usually at it's lowest this time of year. Good
luck Larry

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